Activated aluminum tracer tag

ABSTRACT

A method of treating a piece of aluminum and detecting the presence of the treated aluminum in an assembly without disassembly of the parts. The small piece of aluminum is made radioactive by irradiation with x-rays of at least 23 MeV to produce the reaction 27Al( gamma , Alpha n)22Na. The presence of the aluminum part is determined by isotope decay which emits a positron. The positron subsequently annihilates emitting two 180* correlated 0.511006 MeV gamma rays. These gamma rays are detected in coincidence by use of a pair of detectors and associated coincidence and counting electronics.

United States Patent 1 Murray 1 1 Dec. 2, 1975 I54] ACTIVATED ALUMINUM TRACER TAG 3,781,556 12/1973 Taylor ct al. 250/302 [75] Inventor. lznneth M. Murray, Falls Church. Primary Examiner-Archie R Borchelt Attorney. Agent, or FirmR. S. Sciascia; Arthur L. [73} Assignee: The United States of America as Branning: Melvin L. Crane represented by the Secretary of the Navy, Washington, DC. 57] ABSTRACT [22] Filed; June 20, 1974 A method of treating a piece of aluminum and detecting the presence of the treated aluminum in an assemlzll Appl' 481'289 bly without disassembly of the parts. The small piece of aluminum is made radioactive by irradiation with [52] US. Cl. 250/303; 250/302; 250/304; x-rays Of at least 23 Me\/ to produce the reaction 250/492 R Al(y ozn) Na. The presence of the aluminum part is [51] Int. Cl. G01T l/l6; G21H 5/02 rmined by isotope decay which emits a positron. [58] Field Of Search 250/302, 303, 304, 308, The positron subsequently annihilates emitting two 250/492 493 180 correlated 0.511006 MeV gamma rays. These gamma rays are detected in coincidence by use of a [56] R f ren e Cited pair of detectors and associated coincidence and UNITED STATES PATENTS counting electronics.

3,254,213 5/1966 Fanning ct al. 250/304 3 Claims, 2 Drawing Figures PM TUBE HV POWER SUPPLY gO POSITRON SOURCE U.S. Patent Dec. 2, 1975 3,924,125

' i v I .1 f l5 1 -lI Io I4) HV POWER SUPPLY 60 2| PosITRoN souRcE 23 R 22 (N 24 PM TUBE f PM TUBE PRE- J 26\/ PRE- AMPLIFIER AMPLIFIER 27 28 SINGLE CHANNELU \fSlNGLE CHANNEL ANALYZER /\29 ANALYZER L TIME TO HEIGHT I CONVERTER SINGLE CHANNEL I ANALYZER FIG 2 2 CouNTER 3 TIMER ACTIVATED ALUMINUM TRACER TAG BACKGROUND OF THE INVENTION This invention relates to a method of treating a small piece of aluminum for detection in the presence of other materials.

Heretofore methods have been set forth for detection of the presence of a hidden explosive by radioactive tagging the explosive. This method involved adding an additive to the explosive or by securing a radioactive material to thedet onator or blasting cap or to some other portion of the body of the explosive. Suitable materials include Na or Al each of which have relatively long half lives. For shorter half-life materials, As or Rh may be used. Such method and materials have been setforth in US. Pat. No. 3,255,352.

Aluminum is a common material used in a vast num ber of ways. It is often desirable to know if some small piece is present in agiven assembly without involving disassembly. Aluminum is quite transparent to X-rays and not of sufficient uniqueness in acoustical properties to be amenable to these techniques of imaging or location. Hence, it is often nearly impossible to detect the presence of a small aluminum part in the presence of the structure containing it.

SUMMARY OF THE INVENTION The method of this invention is carried out by exposing a small aluminum part to high energy electromagnetic radiation such as X-rays of greater then 23 MeV to produce the reaction Al('y,om) Na. This isotope decays with a 2.602 year half-life emitting a positron which subsequently annihilates emitting two 180 correlated 0.51 1006 MeV gamma rays. The gammas are detected by a coincidence detector. Thus, the aluminum part is detected by use of a coincidence detector which detects the gamma rays in the presence of other background radioactivity which may be present.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates part of a primer of a naval shell. This primer contains an aluminum shear pin which is to be detected.

FIG. 2 is a block diagram of a coincidence counting system used to detect positrons from the Na activity in the shear pin.

DESCRIPTION OF THE INVENTION In setting forth the method of this invention, reference will be made to the usefulness of the method in determining the presence or absence of an aluminum shear pin 11 in the primer of a naval shell such as shown in a partial cross sectional view in FIG. 1. As shown, the primer includes a steel casing and 13, which contains therein the primer charge 14, a percussion cap 15, the shear pin 11 and the firing pin 12 which is prevented from accidental firing by use of the aluminum shear pin which passes through the firing pin. Once the shell is assembled with the primer in place, one cannot tell through a visual inspection whether or not the shear pin is in place. If the shear pin is not in place, through being left out, it could result in an undesired explosion. Therefore, it is very important for one to determine whether or not a shear pin has been installed in the primer assembly.

A plurality of the aluminum shear pins may be irradiated at the same time with sufficient electromagnetic 2 radiation to produce a useful level of radioactive activity per pin. It was been determined that 1,000 pins having a diameter of 40 mils, with a length of one half inch, can be irradiated with a 50 MeV energy X-ray beam with an average current of 50 micro amps for 10 hours to produce a sufficient activity (i.e. 10' curies of Na). It has been further determined that 10,000 pins may be irradiated for 30 hours with the energy as above to obtain about 10* curies of Na per pin. In about 10 years, there will still be at least 7 X 10 curies of Na per pin, which is detectable by a coincidence counter.

Irradiation of the aluminim pieces produces the reaction Al('y,an) Na. The activity produced, Na, has two very desirable properties, a long half-life of 2.6+ years and the Na decays to a stable Ne by emitting a positron. The positron mode of decay allows a uniquely low level of activity to be detected. The emitted positron always finds an electron in the surrounding material and since these are antiparticles of each other they annihilate each other emitting two 0.51 1* MeV photons in dimetrically opposite directions. These are detected by a coincidence detector system, which is able to discriminate against all other background radioactivity to a very high degree. Thus, an extremely low level of activity may be detected and therefore only a small activity in the shear pin is required.

In carrying out the invention, one wants the lowest background obtainable with as high a pin activity as possible, within a safe range. Activities less than 0.1 milli curies need no licensing or special handling, also activities less than 5 X 10 curies are not considered to be radioactive sources, and for shipping purposes there are no restrictions, for activities less than 1 milli curie, if the dose level at the surface of the package is less than 0.5 MR/hr. The above described irradiation levels for the time period used during irradiation produces activities which are below these safety limits after an initial cooling period of 10 to 15 days.

Coincidence detector systems are well known in nuclear physics research. FIG. 2 illustrates a block diagram of a suitable system. As shown, the system comprises a power source 20 in combination with two NaI(T1) scintillation counters 21,22. These produce a pulse of light proportional to the energy deposited when a gamma ray interacts in them. The light pulse is converted to an electrical pulse by the photomultiplier tube 23, 24 the output of which is then amplified. The amplifier 25, 26 differentiates the pulse into a bi-polar pulse whose voltage crosses the base line (zero voltage line). Since pulses of all amplitudes cross the base line at exactly the same time this cross over is detected and used to generate a timing pulse. The amplitude of the pulse is also detected by a single channel analyzer 27, 28 so that the timing pulse is only provided when the pulse falls within the amplitude limits set by the single channel analyzer. Rather than adjust the instrumental delays such that the two pluses coming from a true coincidence (between the emitted 0.511 MeV photons) arrive at some point at exactly the same moment in time, one finds it easier from an operational point of view to deliberately delay one pulse with respect to the other and then to put these into a time to amplitude convertor 29. This is circuitry which, as the name implies, puts out a pulse whose amplitude is proportional to the time difference between two input pulses. The output pulses will group in amplitude around an amplitude determined by the delay introduced and will have a scatter proportional to the time jitter" in the detection system. The jitter is 35 nanoseconds for this system and is due mostly to differences in collection times for the electrons from the photocathodes in the photomultiplier tubes.

Another single channel analyzer 31 is used to select only those pulses which have the right time delay within the limits determined by the above mentioned jitter. The output of this last single channel analyzer is fed into a counter/timer 32 which is used to determine the count rate. The prototype coincidence counting system constructed for the shear pin detection has a background counting rate of a few tenths of a count per minute. This means that only a very small amount of Na need be present in a shear pin for one to be able to detect its presence.

It is seen by the above that a piece of aluminum may be irradiated such as to produce within it a level of radiation which is not harmful to personnel who may handle it while being sufficient to be readily detectable. Thus, aluminum parts may be detected in the presence of other masking materials.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by letters patent of the United States is:

l. A method of treating an aluminum element for the purpose of detecting said treated aluminum in an assembly in the presence of other masking materials; which comprises,

irradiating said aluminum part with X-rays with an energy of at least 23 MeV to produce the reaction Al('y,ozn) Na.

2. A method as set forth in claim 1; in which,

at least one aluminum part is irradiated for a period of time and a beam intensity sufficient to produce a level of Na activity in each and every part irradiated of at least 10 curies.

3. A method as set forth in claim 1; in which,

said aluminum part is irradiated for a period of 1 hour at 50 MeV using a 50 micro amp. beam. 

1. A METHOD OF TREATING AN ALUMINUM ELEMENT FOR THE PURPOSE OF DECTING SAID TREATED ALUMINUM IN AN ASSEMBLY IN THE PRESENCE OF OTHER MASKING MATERIALS; WHICH COMPRISES, IRRADIATING SAID ALUMINUM PART WITH X-RAYS WITH AN ENERGY OR AT LEAST 23 MEV TO PRODUCE THE REACTION 27AL(Y,AN)22NA.
 2. A method as set forth in claim 1; in which, at least one aluminum part is irradiated for a period of time and a beam intensity sufficient to produce a level of 22Na activity in each and every part irradiated of at least 10 10 curies.
 3. A method as set forth in claim 1; in which, said aluminum part is irradiated for a period of 1 hour at 50 MeV using a 50 micro amp. beam. 